Importing external python module into bge
Use pip to install it! In the blender polder you can navigate to 2.78/bin/python(.exe)
Run that and you have a typical python shell from which you can install pip the usual way (ie getpip.py). Then you can install whatever modules you like.
I have used this method to intall lxml for bge use. And I have used PIL inside blender (installed with a different method though)

When exporting, all python libraries are automatically copied.

But as monster said, first investigate the bge.texture module. You can merge images, set individual pixels etc. The only thing you can't do is resize them (as fas as I know)